U.S. Pat. No. 4,676,887 discloses a significant advance in the refining of petroleum hydrocarbons to product motor fuels and other products. The process described in U.S. Pat. No. 4,676,887 operates by hydrocracking a highly aromatic feed which is produced by catalytic cracking of a suitable petroleum fraction, usually a vacuum gas oil. During the catalytic cracking the dealkylation processes characteristic of the catalytic cracking process remove alkyl groups from the polyaromatic materials in the feed to produce the gasoline as the main product together with various other higher boiling fractions. A highly aromatic distillate fraction formed in the cracking and boiling from about 400.degree. F. to 750.degree. F. (about 205.degree. to 400.degree. C.), generally referred to as cycle oil, forms a preferred feed for the subsequent hydrocracking step which converts the bicyclic aromatics (naphthalenes) in the oil under relatively mild conditions to monocyclic aromatics in the gasoline boiling range. In this way, the cycle oil from the cracking process is converted to a gasoline range product, which, being highly aromatic, has a high octane value and can therefore be incorporated directly into the refinery gasoline pool without the need for subsequent reforming. A notable advantage of the process is the use of relatively mild conditions e.g., hydrogen pressure under 1000 psig, (about 7000 kPa abs) and moderate conversion coupled with an acceptably low catalyst aging rate so that long cycle durations may be obtained. Reference is made to U.S. Pat. No. 4,676,887 for a detailed description of the process.
As mentioned above, a cycle oil from the catalytic cracking step is used as the feed to the hydrocracking step and usually, a light cycle oil boiling approximately in the range of 400.degree. to 700.degree. F. (about 205.degree. to 400.degree. C.) is suitable. However, if a light cut-light cycle oil with an end point of not more than about 650.degree. F. (345.degree. C.), preferably not more than about 600.degree. C. (about 315.degree. C.) is used it is possible to operate at rather higher conversion levels without a concomitant increase in hydrogen pressure while still maintaining an acceptable aging rate in the catalyst. In addition, the octane rating of the hydrocracked gasoline is higher. Thus, by suitable choice of the hydrocracker feed, an extended range of operating conditions may be utilized while improving product quality. The use of the light cut light, cycle oil in this process is disclosed in U.S. Pat. No. 4,738,766, to which reference is made for a detailed disclosure of the process.
Because the hydrocracking is operated under relatively low hydrogen pressure, conversion is maintained at a relatively moderate level in order to maintain catalyst aging at an acceptable rate. One consequence of this is that the effluent from the hydrocracker contains significant quantities of unconverted material i.e. products boiling above the gasoline boiling range. The hydrocracking step has effected a removal of a significant amount of the heteroatom containing impurities in the cycle oil feed and this is reflected by relatively low sulfur and nitrogen contents in the gasoline conversion product as well as in the higher boiling fractions. In addition, some of the higher boiling fractions have undergone hydrogenation to form more readily crackable components, and for this reason a useful aspect of the process is in the recycle of the unconverted hydrocracker bottoms to the catalytic cracking unit. A process of this type is disclosed in U.S. Pat. No. 4,789,457 to which reference is made for a detailed description of the process.
In application Ser. No. 07/433,251, filed 8 Nov. 1989 we have disclosed a process for the co-production of high octane gasoline simultaneously with a high quality fuel oil useful as a blending stock for road diesel fuel. In the process disclosed in Ser. No. 07/433,251 the cycle oil from the catalytic cracking unit is hydrocracked under moderate pressure conditions to produce the high octane gasoline which is separated in the unit fractionator. Also separated in the fractionator is a heart cut distillate fraction, typically a 420.degree.-480.degree. F.(215.degree.-250.degree. C.)fraction, which is recycled to the hydrocracker. As noted in Ser. No. 07/433,251, the distillate fraction which boils just above the gasoline boiling range has a lower octane number than the hydrocracked gasoline product as well as a cetane number hich disqualifies it from use as a road diesel fuel. Although, with these properties, it might seem to be a rather unpromising material, a closer study of its composition showed that it possessed considerable potential for further processing. This fraction is rich in hydroaromatics which are the precursors of the hydrocracked, aromatic gasoline and therefore could be subjected to further ring opening reactions to increase the yield of gasoline as well as the cetane rating of the distillate by removal of the hydroaromatic components. In the process described in Ser. No. 07/433,251, the portion of the hydrocracked product which contains these hydroaromatic components is recycled to the hydrocracker for conversion to gasoline in a second pass through the hydrocracker. Reference is made to Ser. No. 07/433,251 for a detailed description of the process and its operation.